1. Field of the Invention
The present invention relates to a management scheme for character data in an FM teletext broadcast receiver, especially data in a page memory.
2. Description of the Prior Art
In FM teletext broadcasting, character data is broadcast by using the vertical blanking interval of a conventional FM audio broadcast television signal. On the receiver side, the received character data is decoded according to the FM multiplex broadcast standards to display the resultant data on the liquid crystal display panel shown in FIG. 1 or the like. The display panel includes a header portion for displaying 31 characters (=31 columns.times.1 row) in a small character size, and a text portion for displaying 30 characters (15 columns.times.2 rows) in a standard character size and 2 characters (1 column.times.2 rows) in the small character size.
FIG. 2 shows the arrangement of an FM teletext broadcast receiver. An FM signal received by an antenna (not shown) is amplified by an FM tuner 101. The audio signal is detected and amplified by an audio signal processing section (not shown) to output a sound. A data receiving section 102 extracts only character data from the FM signal. A control section 103 receives the character data from the data receiving section 102 and stores/holds it in a RAM (random access memory) 104. When the character data is to be displayed, the control section 103 loads the character data from the RAM 104 and decodes it according to the FM multiplex broadcast standards. The control section 103 loads the font data of the decoded character data from a character ROM 105, and outputs it to a display section 106, thereby displaying the character data.
The character data programs provided by FM teletext broadcasting are classified into various programs, e.g., a news program, a weather forecast program, and a traffic information program. The user can select/display each program. The character data in each program is repeatedly retransmitted (broadcast), but no retransmission cycle is specified. For this reason, if the user requests a given program, and needs to wait for the next transmission of the program, the response for display to the request becomes slow. Therefore, the character data in all the programs are preferably stored in a memory.
FIG. 3 shows the structure of page data used to display character data on an FM teletext broadcast receiver. The FM teletext broadcast receiver stores/holds data in the memory in units of pages. Each of page data 108 is constituted by packet data 107, each serving as the minimum unit in the FM teletext broadcast data structure. One page data 108 has several packet data 107. Each page data is variable-byte-length data. One packet data 107 has 22 bytes. Packet data are broadcast at a transfer rate of one packet per 18 msec.
A videotex system, an RDS (Radio Data System), and the like are similar to an FM teletext broadcast system. However, the FM teletext broadcast receiver uses a microcomputer (system control section) with a low processing speed and a small-capacity external RAM for storing data because of the small system size as compared with the videotex system. In addition, since the transfer rate of broadcast data is low, once the user misses desired data, he/she must wait for the retransmission of the data. That is, the response to a request is slow. Furthermore, in the FM teletext broadcast system, since the amount of data handled is large, the utilization efficiency of the memory for storing data must be improved to store data in the memory as much as possible, as compared with the RDS.
The first data memory management method (Japanese Unexamined Patent Publication No. HEI 9-153831) for a conventional FM teletext broadcast receiver will be described with reference to FIGS. 4A, 4B, 5, and 6. FIGS. 4A and 4B are block diagrams for explaining the data memory management method for the FM teletext broadcast receiver. A storage section is constituted by a data storage area A.sub.1 which is divided into areas (to be referred to as blocks hereinafter) each consisting of a predetermined number of bytes and is used to store received character data for FM teletext broadcasting, a state bit table B in which "1" is set to indicate that data has been stored in a corresponding block of the data storage area A.sub.1, and "0" is set to indicate that no data is stored in a corresponding block, and an information table C for storing program numbers, page numbers, and storage block number data to manage the types of data stored in the respective blocks.
FIG. 5 is a flow chart showing a procedure in the first data memory management method for the FM teletext broadcast receiver. FIG. 6 is a flow chart showing a data storage procedure, in which the information table C in FIG. 4B is searched to determine whether received data of a variable length byte size has already been stored in the data storage area A.sub.1 in receiving FM teletext broadcast data (steps S14 and S15 in FIG. 5). If it is determined that the received data is not stored, available blocks in the data storage area are searched out on the basis of the state bit table B (steps S8 and S9 in FIG. 6) in FIG. 4A. The received data are then stored in units of blocks (step S12 in FIG. 6), and the data, in the state bit table B in FIG. 4A, which correspond to the blocks in which the data have been stored are set in a storage disabled state which indicates that data has been stored (step S13 in FIG. 6). Table data based on the received data is formed at the end of the information table C (step S17 in FIG. 5), and the data in the information table C are rearranged in a predetermined order (step S18 in FIG. 5).
The second data memory management method for a conventional FM teletext broadcast receiver to which a page memory function of holding designated specific page data upon completion of an FM teletext broadcast program is added will be described next with reference to FIGS. 7, 8A, and 8B. Referring to FIG. 7, a data storage area A.sub.2 is divided into a program data storage area A.sub.2-1 and a page memory storage area A.sub.2-2. As the page memory storage area A.sub.2-2, an area corresponding to the maximum necessary number of pages for a page memory is ensured. The program data storage area A.sub.2-1 is determined by (total number of blocks in data storage area A.sub.2)-(number of blocks in page memory storage area A.sub.2-2). The state bit table B and the information table C shown in FIGS. 4A and 4B are omitted from the arrangement shown in FIG. 7.
When data is to be registered in the page memory (step S19 in FIG. 8A), table data based on the page to be stored is formed (step S20), and the page data of the page to be stored is copied from the program data storage area A.sub.2-1 into the page memory storage area A.sub.2-2 (step S21).
When deletion of data from the page memory is requested (step S22 in FIG. 8B), table data based on the page to be deleted is deleted from the page memory information table D, thereby deleting the page from the page memory storage area (step S23 in FIG. 8B). In this management scheme, the data storage area is divided into the program data storage area and the page memory storage area in advance in this manner.
According to the above scheme, in the FM teletext broadcast receiver having the page memory function of holding designated specific page data upon completion of an FM teletext broadcast program, when the number of pages stored in the page memory is small, in particular, the utilization efficiency of the memory greatly deteriorates because of the presence of an unused, wasteful page memory storage area.
In addition, since an area corresponding to the maximum number of pages which can be stored in the page memory must be ensured as the page memory storage area, the program data storage area undesirably decreases.
Much program data cannot therefore be stored, and the probability that program data to be displayed has been stored in the program data storage area decreases. Consequently, the program data must be received again, resulting in a decrease in display response speed.